A. Field of the Invention
The present invention relates to a power semiconductor device used in electric power converters. Specifically, the invention relates to an IGBT and such a MOS semiconductor device and the method of manufacturing the MOS semiconductor device.
B. Description of the Related Art
The performance of the IGBT has progressed through many improvements. It has been required for the IGBT in the OFF-state to sustain the voltage of the current to be controlled and to interrupt the current to be controlled perfectly. It has been required for the IGBT in the ON-state to make the current to be controlled flow with a voltage drop as low as possible, i.e., with small ON-state resistance, and to work as a switch that causes a low power loss.
There exists a tradeoff relation between the maximum voltage that the IGBT can sustain, i.e., the breakdown voltage, and the voltage drop caused in the ON-state of the IGBT. In the IGBT exhibiting a higher breakdown voltage, a higher ON-state voltage results. To improve the characteristics in the tradeoff relation to the achievable limit, it is necessary to explore many developments for designing the device structure. These include exploring a structure for preventing electric field localization from resulting in sustaining the applied voltage of the current to be controlled.
Another important index that indicates the performances of the IGBT is the tradeoff relation between the ON-state voltage and the switching losses, especially the turnoff loss. Since the turnoff proceeds more slowly as the ON-state voltage is lower, a larger turnoff loss is caused. If the turnoff loss is decreased, a higher ON-state voltage results. If the tradeoff relation is improved, the performances of the IGBT will be improved.
In order to bring the characteristics in the tradeoff relations described above into the best mutual relationship, it is effective to reduce the carrier concentration on the anode side and to raise the carrier concentration on the cathode side so that the ratio of the carrier concentration on the anode side and the carrier concentration on the cathode side is about 1:5. Moreover, it is effective to elongate the carrier lifetime in an n−-type drift layer as long as possible so that the average carrier concentration in the n−-type drift layer is high. The mechanism for raising the carrier concentration on the cathode side is called the “electron injection enhancement effect” (hereinafter referred to as the “IE effect”).
For the cathode structure exhibiting a high IE effect, a high-conductivity IGBT structure (hereinafter referred to as a “HiGT structure”) has been proposed (cf. Japanese Unexamined Patent Application Publication No. 2003-347549 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-532885. The HiGT structure is formed of a heavily doped n+-type layer inserted in such an arrangement that the heavily doped n+-type layer surrounds a p-type base in a planar-gate structure. Moreover, a carrier-stored trench-gate bipolar transistor structure (hereinafter referred to as a “CSTBT structure”) and an injection enhancement gate transistor structure (hereinafter referred to as an “IEGT structure”) have been proposed for obtaining a high IE effect in the trench-gate structure (cf. Japanese Unexamined Patent Application Publication No. Hei. 8 (1996)-316479 and Omura et. al., “Carrier injection enhancement effect of high voltage MOS devices—Device physics and design concept”, ISPSD '97, pp. 217-220. In the CSTBT structure and IEGT structure, an n-type layer doped more heavily than the n−-type drift layer is inserted into the mesa section between the adjacent trenches. Generally, the IE effect in the trench-gate structure is larger than the IE effect in the planar-gate structure.
To realize an optimum carrier distribution localized on the cathode side, it is effective to narrow the pnp-BJT region and to widen the pin-diode region. In the structures exhibiting an IE effect and proposed so far, the ratio of the pin-diode region is increased and the forward bias in an n+/n−-junction plane also is increased.
The IE effect is enhanced also in the IGBT having a trench-gate structure by reducing the ratio of the pnp-BJT region. To reduce the ratio of the pnp-BJT region, it is effective to bring p-type base layers in some mesa sections into an electrically floating state. The IE effect is also enhanced by deepening the trench to separate the trench bottom from the pn-junction. The IE effect is enhanced also by narrowing the mesa section. It is considered that these structures increase the density of the hole current flowing through the mesa section and intensifies the forward bias caused in the n+/n−-junction plane by the voltage drop.
In the CSTBT structure and IEGT structure that utilize the trench-gate structure and include an n+-type layer doped more heavily than the n−-type layer and inserted into a mesa region between adjacent trenches, a carrier distribution localized on the emitter side is obtained and the characteristics are fairly improved. However, there is room for further improvement. To lower the ON-state voltage, it is effective to further raise the carrier concentration on the emitter side. In other words, the IE effect (injection enhancement effect) obtained in the IGBT so far is insufficient.
The manufacturing process for manufacturing the conventional trench-gate structure is longer and more complicated than the manufacturing process for manufacturing the planar-gate structure. The throughput for manufacturing the non-defective trench-gate structure is lower than the throughput for manufacturing the non-defective planar-gate structure. The manufacturing costs for the conventional trench-gate structure tend to be higher than the manufacturing costs for the planar-gate structure. In order to make the cell structure more minute and meticulous to further improve the device characteristics, the manufacturing costs will soar even higher. In the IGBT having the conventional trench-gate structure, the electric field is liable to localize to the trench bottom and avalanche breakdown is caused easily. Therefore, the breakdown voltage of the IGBT having the conventional trench-gate structure generally is liable to decrease.
In view of the foregoing, it would be desirable to obviate the problems described above. It would also be desirable to provide a semiconductor device that exhibits a high IE effect, exhibits a low ON-state voltage, facilitates preventing electric field localization from causing, and exhibits a high breakdown voltage. It would also be desirable to provide a method for manufacturing the semiconductor device having the favorable features as described above through an inexpensive manufacturing process with a high throughput of non-defective products. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.